54 lines
1.7 KiB
JavaScript
54 lines
1.7 KiB
JavaScript
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// Lazy (async) routes — the v8 answer to choojs/choo#653.
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//
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// A lazy route handler loads its view on first match via a native dynamic
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// import (or any promise-returning loader) and caches it forever after.
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// In the browser, while the view is loading, the handler renders the
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// optional loading view — or the PENDING sentinel, which tells choo to
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// keep whatever is already on screen. When the module arrives the wrapper
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// emits a render and the real view morphs in. On the server, toStream()
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// awaits load() before rendering; toString() refuses lazy routes loudly
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// (the missing server story is what stalled #653 — here both halves ship
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// together).
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import { equal } from './assert.js'
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// Sentinel: "keep the current tree, a view is on its way"
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export const PENDING = { __chooPending: true }
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export default function lazy (loader, loading) {
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equal(typeof loader, 'function', 'choo.lazy: loader should be type function')
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let view = null
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let promise = null
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function load () {
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if (!promise) {
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promise = Promise.resolve(loader()).then(function (mod) {
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view = (mod && mod.default) || mod
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equal(typeof view, 'function', 'choo.lazy: loader should resolve to a view function (module default export)')
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return view
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})
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}
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return promise
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}
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function lazyView (state, emit) {
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if (view) return view(state, emit)
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load().then(function () {
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emit(state.events.RENDER)
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}, function (err) {
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// surface load failures instead of hanging on the loading view
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emit('error', err)
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throw err
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})
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if (loading) return loading(state, emit)
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return PENDING
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}
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lazyView.__lazy = true
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lazyView.load = load
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return lazyView
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}
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